Exposure of water purification deficit network in response to nitrogen application intensity.

Ecosystem services are strongly responsive to changes in land use intensity, especially water purification, which is highly sensitive to water pollutant emission. Increased nitrogen (N) application to cropland has potential impacts on the supply and demand for water purification through changes in land use intensity. However, there has been a lack of research focusing on the impacts of cropland N application on population exposure to water purification deficits and their cross-regional delivery network. Taking the Dongting Lake (DTL) Basin as an example, this study explored the spatial pattern of N exposure in the DTL Basin from 1990 to 2015 by integrating water purification deficit and population density. Changes in potential N exposure in 2050 were simulated based on population projection data from the Shared Socioeconomic Pathways (SSP1-5). N export pathways in the DTL Basin were clarified by constructing the N delivery network. The results showed that (1) N exposure increased significantly with increasing N application in DTL Basin. (2) The DTL surrounding area and lower reaches of the Xiangjiang River Basin had high increases of N exposure (50.2 % and 71.6 %) and high increases in N exposure due to increases in N application per unit (N influence coefficients exceeding 0.5). (3) The lower reaches of the Xiangjiang River Basin with the highest population density had the smallest decrease in N exposure (1.4 %-11.1 %) in the SSP1-5 scenarios. (4) During 1990-2015, the increase of N export to the DTL surrounding area was higher in the lower reaches sub-basins of DTL Basin. N application had a stronger impact on N delivery processes in the lower reaches of DTL Basin. Managers should distribute N applications to basins with high N retention and whose N export to the DTL surrounding area is weak. This study confirmed the strong response of water purification deficits and their population exposures to N application, and provided decision-making guidelines for water quality enhancement programs in DTL Basin from a spatial planning perspective.